In this case i have to use 50g cold/active starter and feed it with 100g water and 100g flour before usage, don't I? Then the total of 125g water and 125g flour is substracted from the total weight, correct?That means that i only add 500g flour and 250g water + salt?!

BTW, I would NOT recommend having both starters out at the same time - especially young ones that may still be vulnerable to contamination or cross-contamination. I would focus on one starter at a time.

parallei

Assuming your starter is 50/50 water flour (by weight), you would use 30.71g of your activated starter. It contains the 15.36 g of water and 15.36 g of flour that shows up in the "Total Formula". In this case the "Preferment" would be you starter.

If you where to use a larger amount of "Preferment", say 20% of the total dough weight, then it might make sense to use a bit of your starter (say a tablespoon) to make a "leaven" by adding the bit of starter to a larger amount of equal weights of flour and water and letting this "leaven" become active. The "leaven" would the be used as the "Preferment" in the dough calculator.

If you play with the numbers in the dough calculator, you will get a better sense of how it works. Just remember that the "Preferment" + "Final Dough" = "Total Formula"